Oh man. It might not look like much these days, but before Grand Theft Auto III - this was the sandbox that I spent hours and hours in. Just a great mix of gameplay, amazing levels that are worth exploring in full detail and some great mechanics. I'll be picking this up as soon as possible!
I was hoping for this the moment that I saw Zeus & Poseidon pop up. This game got me into city building in a huge way - it's amazingly detailed without being too deep and allows for a variety of ways to build up your city - you'll find yourself improving each and every time you play. If you have even a passing interest in city building/management I suggest you pick up this game and Zeus & Poseidon as well.
Oh lord did Codename 47 need a sequel. A great concept that didn't exactly work out the way that it was intended due to a few, small errors that added up to make a sometimes frustrating game experience. Silent Assassin on the other hand got everything right.